Abstract The serotonin (5-hydroxytryptamine) content of eight raphe nuclei was measured 9 days after injection of p-chloroamphetamine. 5-Hydroxytryptamine levels were reduced in only the B-9 cell group. The histofluorescence of serotonergic neurons in the B-9 cell group was also studied at two time intervals after a single injection of p-chloroamphetamine. Nine days after the administration of p-chloroamphetamine there was a small decrease both in the intensity of fluorescence and in the number of yellow fluorescent serotonergic cells. After 2 months the drug caused a more marked decrease in the number of viable serotonergic cells. The results are discussed in relation to the mechanism of action of p-chloroamphetamine. It is suggested that following the administration of p-chloroamphetamine retrograde destruction may occur in the B-9 cell group, in contrast to those of the other raphe nuclei, whose cell bodies may possess a sufficient number of collateral processes to resist the slow neurotoxic destruction of their cell bodies.